Dr. Muh. Sarkowi Dosen Tekhnik Geofisika Unila

Kamis, 29 April 2010

Time lapse microgravity study for injection water monitoring of Talang Jimar Field

ABSTRACT

Microgravity survey is the geophysical methods to detect the variation of the density in the subsurface; on the other hand injection of water to the reservoir will change the density of some parts of the reservoir. The injection water to reservoir is one of the methods used to drive the oil to flow to the producing well. The Talang Jimar Fields is one of the oil fields located in the South Sumatra Basin of Indonesia. The discovery well in the field was drilled by Dutch Company namely BPM in 1937 in the Talang Akar Formation of Miocene age at the total depth of 1337.6 meters. After produced oil for some years, at present the Talang Jimar field showed the decrease of its production. Therefore some effort to retain its production was done. One of the works was to inject water into the reservoir by some injection wells. This study was carried out in order to monitor the movement of water injection water in the reservoir by observed the microgravity values at different time, and to give a suggestion for future production well location. Time lapse microgravity mapping in the Talang Jimar Fields was carried out three times. The first measurement was carried out on January 2003, the second in May 2003 and the third in September 2003. From those three maps as the result of measurements we can make three different maps of time lapse microgravity anomaly maps. Using these maps we can develop the density distribution map at the depth or reservoir. The development of density distribution map is done by applying the forward and inverse modeling approach. The result shows that the trends of relatively high anomalies are similar to a geologic faults structures pattern as northeast-southwest. It indicates that the faults are sealing for horizontal flow of water, but it could flow vertically. We confirm this indication by forward modeling method supported by geology and well data. As the general structure of the area is graben type, consequently the microgravity anomalies caused by injection water are distributed in the both sides of the graben. Some high microgavity anomalies are found in the southern part caused by existing of some faults trending northeast-southwest. The result of this study gives the understanding to us about the injection water distribution and the role of faults to direct the flow of injection water. Therefore we conclude that the injection well should be placed in the middle part of the graben, on the other hand the development wells are placed in both sides of the graben.

Application of microgravity survey by measuring gravity change in time had been used extensively in many fields. The major causes of gravity change are mass change in reservoir include ground water level change (subsurface) and vertical ground movement (subsidence). While the observed gravity change, called as time-lapse microgravity anomaly, is as superposition of all the causes, hence how to identify each source is very important because some of causes could have similar response. As example is increase of subsurface density (mass) and subsidence.

In order to minimize this uncertainty, time-lapse vertical gradient microgravity associated with its time-lapse microgravity is analyzed. Theoretical background of this analysis is that vertical gradient microgravity value at the surface will be constant if there is only subsidence (no subsurface mass change). Therefore, response of its time-lapse vertical gradient microgravity for subsidence will be zero. In contrast, subsurface density change is indicated by anomaly values that direct proportional with its subsurface density contrast.

To demonstrate this technique, repeat microgravity and vertical gradient microgravity measurement were conducted in Semarang alluvial plain area that has 2 to 17 cm/year subsidence rate and 1 to 5 m/year ground water level change. Their time-lapse microgravity and vertical gradient anomalies indicates existence of ground water decrease, subsidence, and combination between subsidence and tidal flood. This result confirm with elevation change measurement and ground water level change from well data.